Aspects of the disclosure relate to an apparatus and a method for visual gray balance calibration of an imaging system. Other aspects relate to an apparatus and a method for visual gray balance calibration of a printer.
An imaging system may exhibit a drift in the color response of a print engine, either over time or as a result of a number of factors. In order to compensate for the drift, a tone reproduction curve (TRC) may be used for each of the printer colorants. For a CMYK printer, colorants are cyan (C), magenta (M), yellow (Y), and black (B) inks or dyes used to reproduce a color of a given pixel in an original image. A TRC may be implemented as a look-up-table (LUT) that maps input color values to output color values. Input color values may, for example, represent C, M, Y, K intensity values at a particular pixel that are used to control the amounts of colorant applied to a substrate (e.g., a printable sheet such as paper) by an imaging system. Output color values may, for example, represent C, M, Y, K intensity values obtained in a calibration process by measuring the colors printed on a substrate at different patches. The output color values may be labeled according to their deviation from a reference color value, for example, +1% cyan, −1% yellow. The mapping of input values to output values may further include calculating an error value. In some systems, this error value represents, for example, a deviation of the measured output color values from reference color values. The input color values are adjusted in accord with a given algorithm to reduce error values. The adjustment of input values may compensate for a drift in the color response of a printing device.
A printer, one type of an imaging system, may be calibrated by a user in one of two ways—by using color measurement equipment (for example densitometers, spectrophotometers, and/or inline sensors) or by a process of visual adjustment.
Visual adjustment calibration involves the use of the highly sensitive human visual system to determine whether or not a printer is in calibration. There are many methods of visual adjustment calibration including, for example variation methods and reference matching methods. Variation methods generally involve user selection of the most preferred patch or image among patches or images that vary in color attributes such as hue and lightness levels. Reference matching methods involve user selection of a freshly printed color patch that looks the most like the reference color sample.
One reference matching technique involves using a gray (or neutral) patch as the reference, and adjusting colorant (e.g. C, M, Y) intensity values until a good visual match is achieved with respect to the gray color. One technique of visual gray-balance calibration involves the use of a reusable pre-printed calibration sheet as the reference sample. In general, this method relies on visual comparison of neutral gray reference samples and neutral gray color patches formed by a printer. This is done by overlaying the pre-printed calibration sheet, which has punched holes, on a freshly printed target sheet.
A second reference matching technique involves simultaneously printing CMY patches and K patches and selecting a CMY patch that most closely matches the K patch. The printed K colorant is close to gray despite drifts in the color response of a printing device. Therefore, the printed K colorant serves as a reference for gray-balance calibration.